1. Field of the Invention
This invention relates to an adsorption refrigerating apparatus on which refrigerating operation is performed by taking advantage of adsorption and desorption actions of a refrigerant to and from an adsorbent.
2. Statement of Related Art
Worldwide trend toward shortage or depletion of energy resorces is a serious problem particularly for those countries poor in energy resources. Against wasteful use of energy resources, therefore, stringent control is required henceforth.
There exist various kinds of energy resources, but in the status quo, low-temperature heat sources of less than 80.degree. C., for example, cooling water after recovery of high-temperature heat in thermal power plants or heat generated subsidiarily in chemical plants, etc. are discarded without being harnessed effectively because there are problems in efficiency of a recovery apparatus for them and recovery cost.
Further, in the field of utilization technology for solar heat energy which is being developed aiming at aquiring clean energy, it is well known that to utilize a low-temperature heating medium of less than 80.degree. C. which is easily made available by means of a flat-plate heat collector, as a heat source for air-conditioning operation is most advantageous in terms of apparatus cost and running expenditure. In this case, however, if the air-conditioning system is constructed of a conventional absorption refrigerator, because of too low temperature of the heat source, it cannot extend sufficiently capability fulfilling temperature conditions of a general air-conditioning system, i.e. inlet temperature of cooling water: 30.degree. C., cooling inlet temperature: 12.degree. C., and outlet temperature of cooled water: 7.degree. C., and a big-sized construction of the refrigerator and accordingly, steep rise in apparatus cost were unavoidable.
For these reasons, an attempt to incorporate an adsorption refrigerator availing itself of adsorption and desorption actions of a refrigerant to and from an adsorbent such as silica gel, zeolite into air-conditioning system instead of the conventional absorption refrigerator is currently under review.
A known example of an adsorption refrigerator of this type is depicted in FIG. 8. This adsorption refrigerator is constructed of a sideways-elongated cylindrical vacuum vessel (1) sealed therein with a given amount of a refrigerant, internally of which there are horizontally disposed at definite intervals finned heat transfer tubes (2)(2') for routing therethrough a heating medium available from a solar-energy collector, etc. and dish-form vaporizer-condensers (4)(4') rendered integral with linear manifolds (3)(3') for passage of a utilization-side heating medium; a cylindrical shield having an exhalation resistance is installed to encircle the vaporizer-condensers (4)(4'); and a solid adsorbent (7) such as zeolite, activated charcoal, activated alumina or silica gel is filled in the interstices between opposed fins (6) on the outer periphery of the heat transfer tubes (2)(2').
With this apparatus, at desorption run stage when a fluid supplied from a heat source is routed through the heat transfer tubes (2)(2') to heat the solid adsorbent (7) and cause desorption, the refrigerant vapors exhaled from the adsorbent (7) are condensed to deposit on the surfaces of the vaporizer-condensers (4)(4'). On the other hand, at adsorption run stage when a cooling water is flowed through the heat transfer tubes (2)(2') to cool the solid adsorbent (7), the adsorbent (7) adsorbs the refrigerant vapors within the vacuum vessel (1) and the refrigerant liquid on the surfaces of the vaporizer-condensers (4)(4') in vapor state. When the refrigerant on the surfaces of vaporizer-condensers (4)(4') is vaporized and to be adsorbed, it deprives the vaporization latent heat of them and cools the utilization-side heat transfer medium which routes through the manifolds (3) integrated with the vaporizer-condensers (4)(4'). In this manner, the adsorption and desorption are alternately repeated to cool the utilization-side heat transfer medium which is used for air conditioning of buildings, etc. This type of adsorption refrigerator is, for example, disclosed in Japanese Patent Publication JP 60-36852 (1985) A1.
Generally, in an adsorption refrigerator wherein an adsorbent is used, the shorter the time required for adsorption and desorption of an adsorbent (7) is, the more the refrigerating capability per unit time is increased, and refrigerating capability upon continuous operation is also enhanced vastly. The amount of refrigerant within the vessel (1) (namely, saturated adsorption amount of the adsorbent) is, as referred to above, determined on the basis of the temperature conditions of air-conditioning system when the apparatus is run, i.e. refrigerating capability and setting temperatures, and at the same time, required amount of the adsorbent is also indispensably determined. As a consequence, provided that the amount of adsorbent is determinate, time of the adsorption-desorption cycle, particularly speed of adsorption stage depends largely on the configuration of the heat transfer tube (2) on which the adsorbent is packed and held.
On the vaporizer-condenser side, the ability of retaining condensation of the refrigerant, particularly that of retaining the refrigerant in a homogeneous and thinnest possible liquid film state can speed up the adsorption of the adsorbent (7).
In addition to the adsorption-desorption speed of adsorbent and a refrigerant-retaining ability on the vaporizer-condenser side as described above, excess or deficiency of the refrigerant amount within the vessel further affects largely on the refrigerating capability of an adsorption refrigerator and hence it is crucial to regulate the refrigerant amount.
What is responsible for the excess and deficiency of the refrigerant amount is considered as follows:
An operation example with an adsorption refrigerator will be explained on the basis of a diagram illustrated in FIG. 9 showing properties of an adsorbent.
At the time when desorption is terminated, assuming that the conditions are:
adsorbent temperature: 80.degree. C., PA1 condensation temperature: 30.degree. C., PA1 adsorbent: silica gel, PA1 refrigerant: water,
the specific vapor pressure (P/P.sub.o) is 0.09 as calculated: ##EQU1## At this value, the adsorption amount is 0.072 kg/kg as intrapolated from the graph in FIG. 9 (point A). At the time when adsorption is terminated, assuming that the adsorbent temperature is 30.degree. C. and vaporization temperature 5.degree. C., the specific vapor pressure is 0.2 as calculated: EQU P/P.sub.o =6.54 mmHg/31.8 mmHg=0.2.
Here, the adsorption amount of silica gel to the refrigerant is 0.125 kg/kg as intrapolated from the graph (B point).
Therefore, the amount of refrigerant partaking of refrigeration work (circulation amount of refrigerant) is 0.053 kg/kg (=0.125-0.072).
On the other hand, even if the specific vapor pressure at the time of termination of desorption is the value at point A, assuming that the adsorbent temperature is 30.degree. C. and the vaporization temperature 10.degree. C. when adsorption is terminated, the specific vapor pressure is 0.29 (P/P.sub.o =9.26 mmHg/31.8 mmHg=0.29). Here, the adsorption amount is 0.16 kg/kg (as intrapolated from the graph, point C) and the refrigerant circulation amount is increased to 0.088 kg/kg (=0.16-0.072).
It follows from this that even if the specific vapor pressure when desorption is finalized is the same, rise in the vaporization temperature at the time of termination of adsorption or drop in the adsorbent temperature increases the circulation amount of the refrigerant and accordingly, should enhance refrigeration capability.
However, when the filling amount of refrigerant is predetermined so that the refrigerant may circulate between point A and point B, even if it is tried to operate between point A and point C by altering the setting temperatures, the deficiency in the refrigerant amount will deteriorate refrigerating capability. Conversely, when the filling amount of the refrigerant is predetermined so that the refrigerant may circulate between point A and point C, if the running conditions are altered to those between point A and point B, the refrigerant amount becomes excessive and the surplus refrigerant is deposited on the inner face of the vaccum vessel (1). And the refrigerant, when vaporized, is consumed as an energy for cooling the vacuum vessel (1), thus being fraught with energy loss.
However, the prior art adsorption refrigerator as mentioned above is generally aimed at utilization of relatively high-temperature heat source of 100.degree.-300.degree. C., and can be constructed so that the adsorbent temperature at the time of desorption being terminated may be high and the refrigerant adsorption amount may be large, whereby refrigerating capability as required can be ensured. Accordingly, for the heat transfer tube (2), its fin shape, fin height, etc. are not particularly devised, but a usual finned heat transfer tube which holds a solid adsorbent on the outer periphery thereof is merely used.
If the known apparatus is run by harnessing a low-temperature heat source of less than 80.degree. C., the refrigerant adsorption amount will reduce to a great degree and refrigerating capability per unit time will diminish markedly. Thus, the temperature conditions of air-conditioning system cannot be satisfied.
In order to overcome these problems, it is not impossible to increase packing amount of the adsorbent and number of the heat transfer tube for holding the adsorbent, but this will entail problems of providing a large-sized refrigerator apparatus and high rise in product price.
Another problem with the vaporizer-condensers (4)(4') in a tray form is that they have a small refrigerant-holding area and cannot control the thickness of refrigerant liquid film formed thereon. Hence, by the use of the tray-form vaporizer-condensers, refrigerating capability could not be extended sufficiently.
Further problems with such tray-form vaporizer-condensers is that at desorption run stage, since a temperature difference exists between the inlet and outlet for cooling water on the vaporizer-condenser side, condensation amount of the refrigerant, namely thickness of refrigerant liquid film on the surfaces of the vaporizer-condensers is not uniform over the entire surfaces. In a larger thickness portion of the liquid film than a predetermined thickness, vaporization speed is late and adsorption-desorption cycle time is prolonged, and the refrigerant liquid portion which no longer can sustain itself drops down onto the bottom of vessel and is deposited on its surface. The deposited refrigerant, upon adsorption running, is vaporized to cool the vessel, thus being consumed as a vain energy, and cannot be output as a refrigerating capability, which runs into reduction of efficiency of the refrigerating apparatus as a whole.
Again, in the known adsorption refrigerator, since the refrigerant which is hermetically filled in the vacuum vessel (1) and participates in adsorption and desorption is always contained in a constant amount, there occurs change in the temperature conditions identical to load variation of the adsorption refrigerator. For instance, when an intended setting temperature is adjusted to drop in response to increase in the load on the utilization-side, the refrigerant amount is too deficient to exhibit the refrigerating capability sufficiently and conversely, when decrease in the load on the utilization-side renders the refrigerant amount excessive, the refrigerant liquid film formed on the surfaces of vaporizer-condenser trays is not only thickened more than the value predetermined, which runs into reduction of heat transfer coefficient, but also drips down on the bottom of the vacuum vessel (1) and the refrigerant liquid dropped causes energy loss upon adsorption, cooling the vessel itself.